Spectral Water Quality Sensors: What They Measure and How to Choose
UV-VIS photometers, spectrometers and fluorometers: reagent-free, continuous optical water analysis without sample preparation.
Sensor Selection by Parameter
Spectral (optical) water quality sensors are immersed directly in water and measure light absorption or fluorescence; there are no reagents, grab sampling or laboratory delays. Start by identifying the parameter you want to measure:
- Nitrate, nitrite: NICO (nitrate only), OPUS aero (nitrate/nitrite in aeration basins), OPUS (nitrate, nitrite and organic load). Details: nitrate sensor selection.
- SAC254, UV transmittance, COD / BOD / TOC equivalent: LISA UV (T100 4 s, at most 1 W), NICO plus (combined with nitrate), OPUS (full spectrum). Details: online COD measurement.
- Oil-in-water and PAH: enviroFlu, microFlu V2 HC (UV fluorescence). Details: oil-in-water measurement.
- Chlorophyll-a, blue-green algae, CDOM, tryptophan: microFlu V2. Details: chlorophyll sensor.
- pH, conductivity, turbidity: TriOS TW series (drinking water flow-cell sensors).
TriOS Optical Water Quality Sensors
Values are taken from the TriOS product catalog (2022) and the NICO plus (11/2025), LISA UV (12/2025), microFlu V2 (12/2024) and enviroFlu (04/2025) documents. Ranges apply to the stated optical path length.
| Sensor | Principle and light source | Measured parameters | Example measuring range | Response time | Power | Output |
|---|---|---|---|---|---|---|
| NICO | UV photometer; xenon flash lamp, 4 photodiodes + filter | NO3-N, NO3, NOx-N, NOx | 1 mm: 0.5–60 mg/L NO3-N 10 mm: 0.05–6 mg/L NO3-N | T100 20 s | at most 7 W | Ethernet, RS-485 Modbus RTU |
| NICO plus | UV photometer; xenon flash lamp, 4 photodiodes + filter | NO3-N, NO3, SAC254, UVT254, COD, BOD, TOC, DOC equivalents, turbidity, TSS | 10 mm: NO3-N 0–5 mg/L; SAC254 0–60 1/m; CODeq 0–88 mg/L; BODeq 0–29 mg/L; TOCeq 0–35 mg/L | T100 40 s | at most 7 W | Ethernet, RS-485 Modbus RTU |
| LISA UV (LISA-254) | UV-LED photometer; 254 nm and 530 nm (turbidity) LED, photodiode | SAC254, COD, BOD, TOC equivalents, UVT, 530 nm turbidity | 1 mm: SAC254 5–1500 1/m; CODeq 8–2200 mg/L 10 mm: SAC254 0.5–150 1/m; CODeq 0.8–220 mg/L | T100 4 s; measurement interval at least 2 s | at most 1 W | Ethernet, RS-232/RS-485 Modbus RTU or 4-20 mA |
| OPUS | Spectrometer; xenon flash lamp, 256 channels, 200–360 nm, 0.8 nm/pixel | NO3-N, NO2-N, COD, BOD, DOC, TOC, TSS equivalents, SAC254 | 1 mm: NO3-N 0–100; CODeq 0–2200 mg/L 10 mm: NO3-N 0–10; CODeq 0–220 mg/L | T100 2 min | at most 8 W | Ethernet, RS-232/RS-485 Modbus RTU |
| OPUS aero | Spectrometer; 200–360 nm, 256 channels | Nitrate or nitrate + nitrite (depending on calibration) | 1 mm: NO3-N 0.7–36 mg/L | T100 2 min | at most 8 W | Ethernet, RS-232/RS-485 Modbus RTU |
| enviroFlu-HC | UV fluorescence; xenon flash lamp (254 nm), photodiode (360 nm) | PAH, oil-in-water | PAH 0–50 / 0–500 ppb (HC 500); 0–500 / 0–5000 ppb (HC 5000) | at most 10 s | at most 3.5 W | RS-232, 4-20 mA, 0-5 V; MB: RS-485 Modbus RTU |
| microFlu V2 | Fluorescence; LED + filter, photodiode | Chlorophyll-a, blue-green algae (phycocyanin), CDOM, tryptophan, rhodamine, fluorescein; V2 HC: PAH and oil | Chlorophyll-a 0–200 µg/L; CDOM 0–500 µg/L; HC: PAH 0–5000 ppb | T90 6 s | typical at most 0.6 W | RS-485 Modbus RTU, 4-20 mA |
| TW series | Electrochemical (pH, conductivity) and nephelometric (turbidity); flow cell | pH, conductivity, temperature; turbidity | pH 0–14; conductivity 0–5000 µS/cm; turbidity 0–40 FNU (TW Turb-40, ISO 7027) or 0–40 NTU (TW Turb-W40, EPA 180.1) | pH/EC T90: 15 s (rising), 20 s (falling) | — | Ethernet, Modbus |
Three Optical Measurement Principles
UV photometer
Measures light attenuation in water at specific wavelengths (for example, 254 nm in the LISA UV). Nitrate and organic matter strongly absorb light in the UV region. A second wavelength (530 nm in the LISA UV) measures and compensates for turbidity. It is fast and low-power, targeting a focused set of parameters.
Spectrometer
OPUS and OPUS aero scan the 200–360 nm range across 256 channels to acquire the full absorption spectrum. Nitrate, nitrite, organic matter and suspended solids produce distinct curves across the spectrum, allowing the sensor to resolve these components individually. It is slower (T100 2 min) but delivers multiple parameters simultaneously.
Fluorescence
Water is excited at a specific wavelength; a target compound (PAH, chlorophyll-a, CDOM) emits light at a longer wavelength, and this emission is measured. It is selective at very low concentrations. Example: enviroFlu excites at 254 nm and measures at 360 nm; microFlu V2 uses 470 nm excitation and 685 nm emission for chlorophyll-a.
Sum parameters such as COD, BOD and TOC are not measured directly; they are calculated by correlation from UV absorbance and are designated as equivalents (CODeq, BODeq, TOCeq) in TriOS documentation. Because COD and BOD values depend on sample composition, readings should be verified against laboratory results from your own water during commissioning, and an application-specific correlation should be defined if necessary.
Which Sensor for Which Plant?
Wastewater treatment plant
LISA UV or OPUS for organic load (COD, BOD, TOC equivalent) at influent, biological treatment and effluent points; OPUS aero for nitrate/nitrite in the aeration basin. OPUS or NICO plus for simultaneous monitoring of nitrate and organic load in discharge monitoring.
Drinking water
Long optical path NICO and NICO plus measure low nitrate levels; LISA UV reports SAC254 and UV transmittance and is also listed by TriOS for monitoring UV disinfection systems. TW series flow-cell sensors are available for turbidity, pH and conductivity. enviroFlu is used for hydrocarbon contamination.
Surface water, reservoirs and environmental monitoring
microFlu V2 for chlorophyll-a, blue-green algae and CDOM; NICO or NICO plus for nitrate. Multiple sensors are combined at a single station with a TriBox controller.
Refinery, cooling water, marine
enviroFlu or microFlu V2 HC for heat exchanger leak detection and oil monitoring in process water; EGC Water Analyzer for marine exhaust gas scrubber washwater. For details, see our oil-in-water measurement page.
Common Platform: Mounting, Cleaning, Controller
Mounting
- IP68 (NEMA 6P); sample and ambient temperature +2…+40 °C (for specified accuracy); flow velocity 0.1–10 m/s.
- Pressure: 30 bar with SubConn, 3 bar with fixed cable, 1 bar (2–4 L/min) inside FlowCell.
- Power supply 12–24 VDC (plus/minus 10%); body in stainless steel (1.4571 / 1.4404) or titanium (3.7035).
- TW series sensors have an IP30 protection rating and operate in a flow cell.
Cleaning and maintenance
- Nano-coated optical windows; typical inspection at most 0.5 hours per month; calibration/maintenance interval 24 months.
- AirShot2: 10-second compressed air blast, up to every 5 minutes, at most 7 bar; operates with TriBox3.
- FlowCell FC 48/10 USC: ultrasonic cleaning for sensors with an optical path of 10 mm and below.
Controllers: TriBox3 and TriBox mini · Brand: TriOS · Guides: nitrate sensor · oil-in-water measurement · chlorophyll sensor · online COD
Frequently Asked Questions About Spectral Water Quality Sensors
What optical sensors measure, equivalent parameters, turbidity, maintenance and controllers.
What does a spectral water quality sensor measure?
It measures how water absorbs light or emits fluorescence. UV-VIS photometer and spectrometer types (TriOS NICO, NICO plus, OPUS, OPUS aero, LISA UV) report nitrate, nitrite, SAC254, UV transmittance, and derived equivalents of COD, BOD, TOC, DOC and total suspended solids. Fluorometer types (enviroFlu, microFlu V2) measure oil-in-water and PAH, chlorophyll-a, blue-green algae, CDOM, tryptophan and tracer dyes.
What is the difference between a spectral sensor and chemical analysis?
A spectral sensor uses no reagents, takes no sample for wet chemistry, and delivers a reading within seconds to minutes. Parameters such as COD, BOD and TOC are not measured directly; they are calculated by correlation from UV absorbance, which is why TriOS refers to them as equivalents (CODeq, BODeq, TOCeq) and notes that COD and BOD values depend on sample composition. In the LISA UV, an application-specific correlation is defined. Comparing readings against laboratory results during commissioning is good practice.
What is the difference between a photodiode photometer and a spectrometer?
NICO, NICO plus and LISA UV measure specific wavelengths using filtered photodiodes; they are simple, fast and economical (response time T100 is 4 seconds on the LISA UV). OPUS and OPUS aero scan the 200–360 nm range with a 256-channel spectrometer; they separate multiple components (nitrate, nitrite, organic load, suspended matter) from the same spectrum, with a T100 response time of 2 minutes.
Does turbidity interfere with the measurement?
All photometer and spectrometer models perform turbidity compensation: the LISA UV uses a second channel at 530 nm, while the NICO and OPUS families use their own optical methods. Fluorometers (enviroFlu, microFlu V2) do not have built-in turbidity compensation; in turbid water, a separate turbidity sensor or another sensor with turbidity compensation is required.
Under what conditions do the sensors operate?
Submersible TriOS sensors have an IP68 (NEMA 6P) protection rating, a sample and ambient temperature range of +2…+40 °C for specified accuracy, and a sample flow velocity of 0.1–10 m/s. Pressure rating is given as 30 bar with a SubConn connector, 3 bar with a fixed cable, and 1 bar (2–4 L/min) in a flow cell. The body can be selected in stainless steel (1.4571 / 1.4404) or titanium (3.7035); titanium is required for the enviroFlu in seawater.
How much maintenance is required?
TriOS specifies a typical inspection effort of at most 0.5 hours per month and a calibration/maintenance interval of 24 months. Optical windows are nano-coated. The AirShot2 compressed-air system cleans with a 10-second blast and can be triggered as frequently as every 5 minutes; for sensors with an optical path of 10 mm and below, the ultrasonically cleaned FlowCell FC 48/10 USC is available.
Can I combine multiple sensors on a single controller?
Yes. The TriBox3 controller connects 4 TriOS digital sensors via M12 connectors and provides 6 outputs of 4-20 mA, 1 SPDT relay, Modbus TCP, WiFi and 2 GB microSD data logging. The TriBox mini is a compact controller for 2 sensors with 2 outputs of 4-20 mA and a 3.5-inch display.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
Planning a project? Send us your flow rate, feed water source and target water quality to [email protected] — our engineers will reply with a technical proposal.